This invention relates to feedback control loop systems. More particularly, it relates to feedback control loop systems for controlling elements which have a variable gain and frequency response and which produce an output signal proportional to the element gain and its frequency response. A common-mode gain is applied to an element feedback loop, inversely, to approximately equalize loop-gain and bandwidth in the element's feedback loop over the operating element-adjusted frequency range.
In most feedback control systems, the basic problem to be solved is generating and feeding back an error correction signal whose parameters are independent of any effects of gain or frequency response by the element to be controlled. For a control system in which the element under control produces a variable frequency response, it is desireable to generate a controllable "zero" to frequency match/cancel an independent, varying "pole" (such as reflected from varying load inertia) so as to provide a constant performance control feedback loop. According to a prior control system of the same inventor, as described in U.S. Pat. No. 4,839,573 issued Jun. 13, 1989, an element feedback loop is implemented for generating a compensating signal which adjusts the in-series "zero" in order to cancel a varying "pole" in the control loop's feedback response loop-gain, GH, where the varying "pole" results from varying load inertia reflected into the circuit. "Zero" adjustment is achieved by inversely varying the resistance of a variable resistive element (i.e., voltage-controlled resistor or resistor network). For "pole" and "zero" variations over a range of 100:1, the resistance control transmission becomes highly nonlinear. In particular, the prior element feedback loop adjusted a parameter (i.e., resistance) of a control element (adjustable resistive element, such as voltage controlled resistor or FET) to contribute to the determination of the frequency position of a "zero" used to offset an independent, variable, load-associated "pole." However, the frequency response of such control element is highly non-linear. As a result, the loop-gain and bandwidth of the element feedback loop vary non-linearly with changes in the system frequency range and dynamic load response. Accordingly, a linearized method and apparatus is needed within the "zero" element's feedback loop for keeping the loop-gain and bandwidth of such loop relatively constant over a wide range of "pole" (and "zero") frequencies, thereby automatically regulating loop-gain of the second or element feedback loop under dynamic response conditions.